The invention concerns a modularly constructed socket for a ball-joint prosthesis for insertion in bone tissue.
Modularly constructed sockets for ball-joint prostheses usually consist of an outer metal shell, to be inserted in the bone tissue, as the socket shell, and an inner sliding cup placed in the socket shell. This sliding cup can consist of ceramic material, for example, and be fixed to the metal shell, the socket shell, with a conical clamp. Such a socket is described for example in DE 43 35 931 A1. In DE 44 02 875 A1 a socket shell is described in which the inner sliding cup is placed in the socket shell without a conical clamp. So that the sliding cup is protected from rotating or becoming detached, yet can be removed from its seat and exchanged without damage, it is secured by a holding ring in the metal cup, which is screwed to the anterior face of the socket shell.
With the known forms of construction, the outer edge of the metal cup for emplacement in the bone tissue, the edge of the socket shell, is reinforced. The socket shell loses its elasticity. As a result, increased migration of the socket shell in the bone can occur. This leads to gradual loosening of the implant in the bone.
A further problem is matching the sliding cup to the dimensions of the socket shell. The measurements of the socket, essentially the diameter of the ball and consequent internal diameter of the sliding cup, are essentially determined by the age of the patient and his/her bodily stature. With conical clamping fitting in particular, the outer contours of the sliding cup must conform very exactly to the inner contours of the socket shell. Upon any subsequent surgical operation, it is therefore advantageous if there can be retention of at least the shaft in the thigh and the socket shell in the pelvic girdle by the endoprosthesis and only the ball of the ball-joint prosthesis and the sliding cup need to be replaced.
The object of the present invention consists of improving the emplacement of the ceramic sliding cup in a metal socket shell and to facilitate the removal of the sliding cup from the socket shell during any repeat surgery.
This object is achieved with the help of the characteristic features of claim 1. Advantageous refinements of the invention are claimed in the sub claims, to which belongs a piece of apparatus that is advantageously appropriate for manipulation of a sliding cup.
The modularly constructed socket for a ball-joint prosthesis according to the invention differs from the prostheses currently available in that the ceramic cup is not placed directly in the socket shell. The ceramic cup is inserted securely in a ring of biocompatible metal and this ring is placed in the socket shell. For securing the ring to the sliding shell, at least three securing elements protruding radially outwardly, are provided on the ring. These securing elements fit into a recess in the socket shell. The ring does not lie directly on the socket shell, but is separated from it by a gap. As a result of the securing of the cup according to the invention, reinforcing of the socket shell is impeded by the sliding cup, as occurs with currently available conical surface clamping of the sliding cup and socket shell. A possible migration of the socket shell as a result of the reinforcing is accordingly avoided. The socket shell retains its elasticity. Furthermore, it prevents bad or careless positioning of the sliding cup in the socket shell giving rise to flaking of the cup rim, thus making it unusable. The surgeon""s work is made easier, as positioning of the sliding cup in the socket shell under operating conditions is made easier.
The connection between the sliding cup and the ring can be achieved either with conical clamping or shrinking on of the ring. With shrinking on, the ring is warmed and pulled over the sliding cup. On cooling, a shrink connection exists.
To achieve efficient positioning of the sliding cup in the socket shell and at the same time efficient balance within the shell, it is an advantage if the gap between sliding cup and socket shell has the same measurements everywhere. To produce and ensure a constant distance between sliding cup and socket shell, the securing elements have spacers. The spacers can also be constructed so that, by support on the wall of the shell in the region of the securing elements, an equal distribution of the load in the socket shell is ensured. Depending on the size of the socket shell and the sliding cup, the spaces between the sliding cup and socket shell are between about 1 mm and 1.8 mm.
To ensure anchoring of the ring with the sliding cup in the socket shell, the securing elements widen in the radial direction in a wedge shape and the recesses in the socket shell which the securing elements fit match the contours of the securing elements. When a load is placed on the sliding cup, this will seek to push out the socket shell. The wedge shape ensures that this pressing of one out of the other is prevented and the securing elements of the sliding cup ensure that the shell retains its shape as a result of its clamping action, so that dislocation of the sliding cup from the socket shell due to expansion of the latter is avoided.
The load distribution in the socket shell by the securing elements in the socket is further improved if the securing elements narrow in a wedge-shaped fashion in the direction of emplacement in the socket shell and the recesses in the socket shell are matched to the contours of the securing elements. As a result of the shaping of the recesses, a one-sided load distribution via the securing elements in the direction of the meridian is available in the socket shell which has generally the shape of a hemisphere or a section of a sphere. As a result of the wedge-shaped narrowing of the securing elements, not only are load components in the direction of the meridian in the spherical surface but also vertically in the direction of the circumference of the socket shell so that an equilibrium of the load distribution in the region of the recess in the shell is assured.
Because of the various forms of shell, there are various internal diameters for uptake of the cups, which have one and the same internal diameter, as they must fit one and the same ball-joint size. Therefore there is a further advantage to embedding the sliding cups in rings, in that the external diameter of the cup no longer has to conform to various forms of the socket shells. It is sufficient to produce a sliding cup with the predetermined external diameter and fit it to the shell by means of rings of varying thickness. The invention is therefore based on the concept that it is easier to produce a ceramic sliding cup in a single size, because in that way the behaviour of several shapes and the corresponding adjustments of the sliding cups with various tools and various apparatus is superfluous. For a sliding cup destined for a 28-mm ball-joint, the number of sizes of rings required, in which the sliding cup may be embedded, is limited to six and for a 32-mm ball-joint it is limited to three.
A further advantageous refinement of the invention is a tool for manipulation of the sliding cup provided with ring. With this, emplacement of a sliding cup into, and withdrawal from, the socket shell is possible. The tool possesses two components axially mobile with respect to each other, with the first component providing the support sitting on the rim of the socket shell and the second component, providing the force and tension in relation to the manipulation of the sliding cup, being able to be positioned on the sliding cup. The pressing and pulling arrangement is so constructed that, for emplacement of the sliding cup in the shell, it can be positioned on the securing elements to exert, by appropriate manipulation pressure on the securing elements, which are aligned with the socket shell and thereby insert the sliding cup in the socket shell, with the securing elements being pressed into the corresponding recesses.
To make possible release of the cup from the socket shell, the pressing and pulling arrangement is so designed that it can grip the securing elements, and upward pressure can be exerted on the sliding cup by movement of both components of the tool to extract it from the socket shell. While the support arrangement is supported on the rim of the shell, the pressing and pulling arrangement pulls the securing elements out of the recesses in the socket shell. Due to its mode of operation, the tool is comparable with a take-off device from sheet supports, but with the difference that here support does not take place on a centrally arranged sheet with the object to be removed arranged around the point of support, but that the object to be removed is located within the structure from which it must be extracted.